The present invention relates to a thermally printing apparatus having a thermal print head which effects a printing operation through heat transfer.
In a commonly known thermally printing apparatus for printing on sheets of paper of an ordinary kind, i.e., non-heat-sensitive paper, a printing operation is achieved by a thermal print head through a thermal ribbon (accommodated in a cassette) having a thermally transferable ink layer. In such thermal printer, the thermal head is moved for printing along a paper supporting platen while it is held in pressed contact, via the thermal ribbon, with the printing surface of the paper placed on the platen. Consequently, a relative movement between the paper and the thermal ribbon, if any, would cause a friction between an ink layer of the ribbon and the printing surface of the paper sheet, thus resulting in undesired adhesion of an ink to the printing surface of the paper being printed. For this reason, a thermal printing is effected without such relative movement of the thermal ribbon with the paper, and accordingly the printing movements along the platen are adapted to take place in one fixed direction.
When a thermal printing is executed by a thermal print head directly on a sheet of heat-sensitive paper without using the thermal ribbon, there is no such possibility of undesired ink adhesion to the paper as described above. In this case, therefore, a bidirectional printing is preferable to a unidirectional printing, for increased printing efficiency.